The present invention relates to a method and an apparatus for the manufacture of clinker from mineral raw materials, e.g. cement clinker, including substantially removing from the clinker chlorides, sulfur and alkalis (potassium, sodium) in a kiln plant in which the raw materials are preheated, optionally precalcined and then burned and sintered into clinker in the kiln proper, and during which process it is desirable to reduce the contents of harmful compounds, such as chlorides and alkalis, in the manufactured product or to reduce the amount of such compounds circulating in the plant and thereby causing process drawbacks.
U.S. Pat. No. 3,365,521 disclose e.g. a kiln plant for the manufacture of clinker, which plant consists of a rotary kiln having a multistep suspension preheater of the cyclone type and a precalciner coupled before the kiln for expelling CO.sub.2 from the treated raw materials and with a cooler coupled after the kiln for cooling the treated product. The raw materials are counter-currently preheated in the suspension preheater by the hot flue gases from the kiln and/or the precalciner, the flue gases thereby being cooled through heat exchange with the raw materials before the gases are directed to a flue gas filter to be purified before being discharged into the atmosphere.
Generally the raw materials used contain varying but often quite considerable amounts of easily evaporating compounds, such as chlorine, sulfur, potassium and sodium, which are harmful to the product proper and to the manufacturing process. In a conventional kiln plant of the above mentioned type such compounds, even though a major part of them evaporate from the clinker during the sintering process, will be recovered in condensing during the passage of the flue gases up through the preheater or the precalciner and then together with the added raw materials be returned to the kiln. Thus, the chloride and alkali content continues to increase during the manufacturing process until a critical value of the precipitated compounds is reached resulting in the risk of said compounds depositing as blockings and bakings in the riser pipes, the precipitator cyclones and the smoke chamber of the kiln.
As also disclosed in the above mentioned U.S. patent specification, the plant may be provided with a bypass to counteract the above mentioned drawbacks so that part of the kiln flue gases is diverted from the preheater and the precalciner and passed into a separate filter. A bypass for 5-10% of the flue gas is normally sufficient to reduce the amount of chlorine compounds or chlorides in the flue gas, while a reduction of the remainder of the volatile components (e.g. sulfur, potassium, sodium) requires a bypass for 30-60% of the amount of flue gas. The bypass according to said patent specification removes chloride from the kiln system by quenching or cooling the bypassed flue gas with air, thereby causing the chlorides to condense and subsequently to precipitate together with material dust contained in the flue gas in a separate bypass filter and then to be removed from the process. However, the method according to the U.S. patent specification suffers from the drawback that such a plant requires two filters, the primary filter and the bypass filter, and that the material dust precipitated in the bypass filter has to be removed completely from the process together with the harmful, uncondensed chlorides without allowing the reuse of the material dust in the process in an economical manner, thereby improving the operating economy of the plant.
GB No. 1,319,180 discloses a kiln plant having a bypass without a separate bypass filter and in which plant the bypassed flue gas is fed directly into the primary filter and purified in same together with the dust-laden flue gas from the preheater. The bypass contains a cyclone precipitator for precipitating larger particles of material with insignificant chloride and alkali content for reuse in the process, whereas all material dust precipitated in the primary filter has to be diverted from the process due to its contents of chlorides and alkalis, the harmful effects of which prevent it from being reused. Thus, the advantage of using one filter only in this type of plant is compensated by the disadvantage in having completely to remove all the dust precipitated in the primary filter from the process.
Danish patent application No. 2483/86 (corresponding to EP application No. 0209961) discloses a bypass installation in a kiln plant having only one filter also, but said bypass being limited to treating only flue gas from burning clinker having a relatively insignificant amount of added chloride (0.015-0.1 weight percent).
Finally, U.S. Pat. Nos. 3,112,764 and 3,288,450 disclose examples of kiln plants provided with a bypass for part of the flue gas, said bypass containing a precipitator cyclone for precipitating alkalis from the flue gas, the filter dust from the filter of the kiln plant being directed to the precipitator cyclone of the bypass as a coolant causing the alkali vapours in this cyclone to condense upon the dust particles which are subsequently precipitated and discharged completely from the process. The method according to these patent specifications implies the condensation of the alkalis upon the dust particles at a temperature of 500.degree.-700.degree. C., the cooling obtained by means of recirculated filter dust or by adding other fine-grained material being relatively insignificant. Thus, in practice it will prove difficult to obtain a sufficient cut size of the precipitator cyclone through this method to ensure an effective alkali purification of the smoke.
Therefore, it is the object of the invention to provide a method and an apparatus for an improved treatment of bypass flue gas from a kiln plant to remedy the above mentioned drawbacks, and which allow the reuse of precipitated dust both from the primary filter of the plant and from the treatment of the bypass flue gas.